When operators deploy long term evolution, LTE, carriers within the same frequency band as 2G/3G carriers (e.g., code division multiple access, CDMA), interference from a user equipment operating within the 2G/3G radio access technology may interfere with reception of signals by the LTE radio access technology infrastructure. Conversely, interference from a user operating within the LTE radio access technology may interfere with reception of signals by the 2G/3G radio access technology infrastructure. Interference is especially likely when multiple radio technologies co-exist in the same geographical coverage area and have carriers assigned adjacent to each other.
FIG. 1 is a plot of a CDMA signal 2 after despreading and an LTE signal 4 in an adjacent band. As can be seen, a portion 6 of the LTE signal 4 interferes with the CDMA signal 2, and to a lesser extent, a portion 8 the CDMA signal 2 interferes with the LTE signal 4. The problem is more severe on the uplink than the downlink because the user equipment, UE, is often a low cost consumer product and cannot afford to have a transmission filter as sharp as the transmission filter of a base station. The interference problem is exacerbated when an interfering user equipment is far from its serving base station but near the base station receiving the interference and is transmitting at maximum power, while a transmitter of another user equipment of a different technology is in a power limiting situation and is experiencing maximum path loss to its serving base station.
There are existing solutions to address inter-cell interference within a single radio access network, RAN, such as LTE. In such cases, a base station such as an evolved node B eNB, performs interference measurements and reports the measurements to other eNBs, which act to suppress an interference signal. For example, in one solution, a static allocation of a physical uplink control channel, PUCCH, away from an edge of a frequency band may be employed with the disadvantage of fragmenting uplink radio resources without adapting to changing interference conditions. Another solution involves random frequency allocation for physical uplink signal channel, PUSCH, with the disadvantage of not removing interference by the PUCCH. Fractional frequency reuse may be employed, but fractional frequency reuse is not applicable between different co-existing radio access technologies. In third generation partnership project, 3GPP, systems, an X2 interface may be employed to address inter-cell interference, but this is not applicable to interference between two different radio access technologies. Another solution is to rely on a large guard band between two potentially interfering signals, but this is spectrally inefficient.